Regulations require aircraft brakes to be able to handle an aborted takeoff at any moment prior to the plane leaving the runway. Brakes should not exceed a specified temperature, to avoid performance degradation. Current design prohibits an aircraft from taking off if its brakes are too hot (e.g. carbon temperature above 400° C., corresponding to an indicated temperature of about 300° C.). To ensure that the brakes are cool enough even after use during taxiing out to the runway, it is recommended that an aircraft is not dispatched if its brakes are above a predefined temperature (e.g. 150° C., as measured by a brake temperature sensor), which is significantly lower than the maximum permitted take-off temperature and allows for temperature increase during taxi braking.
Application of the brakes of an aircraft can be instigated (demanded) by the flight crew applying pressure to the brake pedals, or by an auto-brake function. When such a braking demand is made, the hydraulic pressure is increased in a number of braking pistons, which apply force to press together the rotors and stators within the brake pack. Assuming that the same braking demand is made of each landing gear (e.g. by the flight crew applying pressure to each brake pedal), the hydraulic pressure in each brake-pack of a landing gear is assumed to be equal.
However; applying equal hydraulic pressure to each brake-pack does not necessarily result in optimal braking performance, or in an optimal turnaround time. Each brake-pack has a brake gain, which defines how much torque is developed per unit of pressure applied. This brake gain can vary from brake-pack to brake-pack (e.g. due to manufacturing tolerances), meaning that each brake-pack may develop a different level of braking torque for the same level of applied pressure.
Furthermore, each brake-pack may be in a different state of wear. Where this is the case, if the torque (and therefore the energy) developed by each brake-pack is identical, each brake-pack will heat at a different rate, with the heating rate being determined by the brake mass. Consequently, each brake-pack will reach a different peak temperature, and will take a different amount of time to cool down to a target temperature suitable for pushback from the stand. If the torque developed by each brake-pack is also different, due to variation in brake gain as explained above, differences in peak temperature and cooling time between the brake-packs may be even more significant.